I have had my petco betta for about six months now(surprise birthday present) and of course i knew nothing about how to care for them. So after receiving my poor lil fishy in a 1/2 gal i upgraded him to a 1.5 gallon bowl. Soon after he started developing fin rot, i treated him and he got better but then a week later he would get it again, this happen for about 2 months or so.
So i finally decided to cycle him a 5.5 gallon hoping this would get rid of his finrot issues. He has been in his newly cycled tank for almost a week and this morning i discovered he had finrot again! I checked my water, pH is 8.0(have very hard water), Ammonia is 0, Nitrate is 0, and Nitrate is 0, temp is 79.
Can anyone tell me what i am doing wrong? I was thinking of dosing with tetracycline because it has worked well before, but will that mess up my newly cycled tank?

I'd also like some pictures to specifically diagnose what it is And don't worry about the pH, the only thing it will do is make the fins curl after a while but it's nothing that will hurt them. Better to have a stable pH than a fluctuating one!

Ah okay, I see the higher pH has done it's work already haha. np though, can you shine a light on him from the front so it illuminates his front more? Camera will be able to focus more and it will light up the dark spots so I can see the rot. It's hard to tell if it's not just his regular color or not, I'm sure you'd know though.

The ends of his fins are naturally grayish black, so it is difficult to see the rot. However i always see the rot because it is a thick black color rather than clear gray and his fins look frayed.
I also thought perhaps he was tail biting, to me it looks like tail biting on the dorsal because the cuts are so clean and fin rot on his back fin because of the black color and it looks like a torn Chinese fan.
For now i guess i will just do a water change and add aquarium salt:) He tends to heal within 3 or 4 days when this happens so that is good:)

Yup that's fine, I also have a DT who is a black/yellow Mustard Gas so his edges are usually black but I can see where there is fin rot like he's dealing with now because it's just a super black edging that he doesn't normally have.

Sometimes fish just get fin rot all the time even though you keep the tank sparkling clean, its just how their system is and usually it continues through their life, sometimes it stops after a few times and for those rare lucky fish, it only happens once or not at all! So you just have to keep medicating him with the salt when it does come about until he's better again. Of course not to be used over 14 days at one time but he should be doing well soon